a) Field of the Invention
The invention is directed to a collector mirror for short-wavelength radiation sources based on a plasma having an optically active mirror surface with high reflectivity for the desired short-wavelength radiation and in which means for cooling the mirror body are provided due to the position of the collector mirror in the immediate vicinity of the plasma which has an extremely high temperature. The invention is preferably applied in radiation sources for large-scale production in semiconductor lithography which emit in the extreme ultraviolet spectral region (EUV region).
b) Description of the Related Art
Plasma-based radiation sources for semiconductor lithography (primarily EUV sources) are essentially thermal radiators which emit their output in a solid angle of 4π sr. The emission is generally not isotropic. The excitation process which can be carried out either by gas discharge (GDPP sources) or by a laser beam (LPP sources) has a low-percentage efficiency in a 2-percent narrow EUV spectrum with a central wavelength between 12.4 nm and 14 nm that is usable for the application. On the other hand, the conversion of the excitation output into a broadband extreme ultraviolet spectrum has an efficiency of several tens percent. The EUV radiation in the range of 1 nm to 100 nm has a large cross section of interaction with material, i.e., this radiation is completely absorbed already over very small path lengths. To compute orders of magnitude, it can be assumed that essentially the entire excitation output of plasma-based radiation sources reaches components of the radiation source through radiation transport and occurs at these components as thermal output.
The collector mirror collects a significant proportion of the radiation of the source (plasma) exiting in the solid angle of 4π sr and projects the radiation in the application band range (wavelength range around 13.5 nm) in an intermediate focus. While glass is suitable in principle for producing collector mirrors for plasma-based radiation sources because it can be produced with excellent surface quality, its thermal conductivity (≈1 W/mK) is too small to use it as a substrate of a collector mirror in an EUV source for semiconductor lithography.
In particular for alternating-layer mirrors with direct (non-grazing) radiation reflection, dissipation of this radiation load and thermal load constitutes a technical challenge because the temperature average of the alternating-layer mirror may not exceed several tens of degrees Centigrade. A degradation in the alternating-layer system takes place even when the mirror is heated (only temporarily) to above approximately 200° C. and leads to a substantial reduction in reflectivity. Transient temperature curves with temporary temperature spikes appreciably above the temporal temperature average of the reflection coating occur due to the pulsed operation of the plasma-based sources.
According to the prior art, collector mirrors are fabricated from substrates, e.g., silicon, having a thermal conductivity appreciably higher than that of glass, and the substrate—as is shown in FIG. 3—is pressed on a heatsink or cooling body. Contact foils, e.g., comprising indium, are often used for this purpose. At the outputs of an EUV source for large-scale production in semiconductor lithography at which several tens of kW excitation output are converted, cooling of this kind is not sufficient for ensuring a long lifetime of the reflection coating of the collector mirror.